1. Field of the Invention
The present invention relates to radio frequency modulators, particularly digital radio frequency modulators.
2. State of the Art
Modulation can be defined as the alteration of some characteristic of a known signal or waveform, i.e., a carrier, as a function of some unknown signal or waveform that conveys information. In radio-frequency (RF) communication systems, the carrier is typically a sinusoid, and there are several methods of modulating the carrier. These include linear modulation, angle modulation, and various types of pulse modulation. Given a sinusoidal carrier described by the equation A(t)cos(.omega..sub.c t+.phi.(t)), there are two parameters, the amplitude and the phase angle, that can be varied in accordance with an information signal. Linear modulation results when the amplitude is varied as a linear function of the information signal. Angle modulation includes phase modulation and frequency modulation. If a term is included in the argument of the sinusoidal function that varies in proportion to the information signal, the result is phase modulation. If the argument is such that the difference in the instantaneous frequency and the carrier frequency is proportional to the information signal, the result is frequency modulation.
Demodulation of RF signals has typically involved a quadrature detector having two branches, an I ("in-phase") branch and a Q ("quadrature" or 90.degree. phase-shifted) branch. In the I branch, a received signal is multiplied by the cosine form of the carrier signal and then passed through a low-pass filter. In the Q branch, the received signal is multiplied by the sine form of the carrier signal and passed through a low-pass filter. Quadrature detectors of this type are linear, well-understood, and almost universally used. To obtain the information signal from the I and Q components produced by the respective I and Q branches of the quadrature detector, signal processing is performed. In particular, the phase of the signal may be obtained by taking the inverse tangent of the ratio of Q to I. The amplitude of the signal may be obtained according to the Pythagorean theorem by taking the square root of the sum of the squares of I and Q. These mathematical operations are non-linear.
Two salient observations may therefore be made concerning quadrature detection. First, detection proceeds in two steps, a first mixing step (to obtain I and Q) that is linear and a second signal processing step to which non-linearities are relegated. Second, a coordinate system conversion is first performed and then reversed. That is, the received signal, which may be readily described in polar coordinates in terms of the desired quantities of amplitude and phase, is first converted to rectangular coordinates by projecting the instantaneous signal vector in polar coordinates onto the X (I) and Y (Q) axes, and is then converted back to polar coordinates to obtain amplitude and phase. Such conversions require circuitry that occupies space and consumes power--both of which may be precious commodities, especially in mobile applications such as cellular telephones, pagers, etc. Such conversions may also entail substantial inaccuracies.
A similar situation exists on the transmit side with respect to RF modulators. That is, amplitude and phase information is originally represented in polar form. I and Q processing is then performed in which amplitude and phase information is encoded as I and Q signals which are then converted back to polar form and summed to form the final output signal. This process is illustrated in FIG. 1. One widely used type of modulation, Phase Shift Keying (PSK), requires the transmit signal to undergo accurate phase shifts and to exhibit good phase stability. PSK is used in GSM cellular telephones, for example. A conventional modulator of the quadrature type, while it exhibits good phase stability, achieves only marginal accuracy. Sophisticated coding schemes may be required to compensate for inaccuracies of the modulator, and performance under noisy conditions may noticeably deteriorate.
Beside conventional quadrature techniques, various other modulation techniques are known. In one such technique, a phase lock loop (PLL) is used to multiply a modulated signal in frequency and phase to obtain a high frequency signal (e.g., 900 MHz). Referring to FIG. 2, a PLL includes a phase detector 201, a loop filter 203, a Voltage Controlled Oscillator (VCO) 205, and a divide by N counter 207. the case of a 900 MHz output signal, a typical divisor might be 64, for example. If a modulated signal cos(.omega..sub.c t+.phi.(t)) is applied to the input of the circuit, then ideally, a modulated signal cos (N.omega..sub.c t+.phi.(t)) is produced at the output of the circuit. The stability of the circuit, however, is poor unless the loop filter is made to have a narrow bandwidth. With a narrow bandwidth loop filter, the circuit is no longer able to track rapid changes in the modulated input signal, producing inaccuracies in the modulated output signal.
To overcome the foregoing difficulty, an arrangement shown in FIG. 3 has been devised. A baseband modulation signal is applied to a Voltage Controlled Crystal Oscillator (VCXO) 309 to produce a modulated signal which is applied to the input of a PLL, as before. An additional feedforward path is used to inject the baseband modulation signal into the PLL at a point past the loop filter. More particularly, the baseband modulation signal is applied to an adjustable gain amplifier 311. The output of the adjustable gain amplifier is applied to an adder 313 situated between the loop filter and the VCO. The gain of the amplifier is adjusted so that the effects of the loop filter in removing some of the modulation is precisely offset by reinjecting the modulation signal. Unfortunately, achieving precisely the correct adjustment is a painstaking manual process. Furthermore, although sufficient precision may be obtained for FM radio communications, the precision required for PSK radio communications is lacking.